1. Field of the Invention
The present invention relates to a focusing apparatus, a focusing method, and an inspection apparatus. For example, it relates to a pattern inspection apparatus that inspects defects of a pattern formed on a mask used for manufacturing semiconductors etc., and to a focusing mechanism or its method used in such an apparatus.
2. Description of Related Art
In recent years, with an increase in high integration and large capacity of large-scale integrated circuits (LSI), a critical dimension of a circuit required for semiconductor elements is becoming narrower and narrower. These semiconductor elements are manufactured by forming circuits by exposing and transferring a pattern onto a wafer by means of a reduced-magnification projection exposure apparatus (a stepper) for example, while using a master pattern (also called a mask or a reticle, and will be hereinafter called a mask) on which a circuit pattern is written, “drawn” or “formed”.
An improvement in yield is crucial in manufacturing an LSI which requires a lot of manufacturing cost. However, as typified by 1 giga-bit DRAM (Random Access Memory), the precision of a pattern, which constitutes an LSI, has been changing from sub-microns to nanometers. One of the major factors that decrease the yield is a pattern-defect of a mask-pattern used in exposing and transferring an ultrafine pattern onto a semiconductor wafer by a photolithography technique. In recent years, with miniaturization of an LSI pattern written on a semiconductor wafer, dimensions, which have to be detected as a pattern defect, are becoming extremely small. Furthermore, inspection of the LSI pattern formed on the actually produced semiconductor wafer is also important, and the measurement to be detected is also becoming very small. Moreover, with development of multimedia, miniaturization of a pattern of a TFT (Thin Film Transistor) or the like formed on an LCD (Liquid Crystal Display) is advancing. Therefore, it is increasingly required that an extremely small pattern-defect should be inspected in a wide range. Therefore, a pattern inspection apparatus that inspects defects of a transfer mask used in manufacturing an LSI, a manufactured semiconductor wafer, a photomask used in manufacturing an LCD, etc. needs to be highly precise.
As to a conventional pattern inspection apparatus, it is well-known that an inspection is performed by comparing an optical image captured by photographing a pattern written on a target workpiece, “object”, or “sample” to be inspected, such as a lithography mask, at a predetermined magnification ratio by using a magnifying optical system, and design data (design pattern data) or an optical image captured by photographing the same pattern on the target workpiece.
For example, the following is known as pattern inspection methods: “die to die inspection” that compares optical image data obtained by photographing identical patterns at different positions on the same mask, and “die to database inspection” that generates design image data, which is converted from the design data as image data, based on writing data (information on design pattern) obtained by converting CAD data used in writing a mask pattern into a format to be input into an inspection apparatus, and compares this generated design image data with optical image data serving as measurement data obtained by capturing an image of the pattern. In such an inspection method of the inspection apparatus, a target workpiece is placed on a stage to be scanned by light flux when the stage moves to perform inspection. The target workpiece is irradiated with light flux from a light source and an illumination optical system. The light transmitted through the target workpiece or reflected by the target workpiece is focused onto a sensor through the optical system. The image captured by the sensor is transmitted to a comparison circuit as measurement data. In the comparison circuit, after performing position alignment of the images, the measurement data is compared with reference data based on an appropriate algorithm. As a result of the comparison, when they are not in accordance with each other, it is judged there is a pattern defect.
In order to increase the preciseness of such a pattern inspection apparatus, it is required to obtain an optical image of large magnification and high resolution in capturing an image of a pattern on a target workpiece to be inspected, such as a mask. Thus, it becomes important to perform optimum focusing of the optical image.
FIG. 17 shows a focusing mechanism applying the conventional optical lever method. In FIG. 17, an optical image of a photographic subject W is focused onto a sensor 501 by an objective lens 502 and an imaging lens 503. In a focusing mechanism 500, the position in the focusing direction of the photographic subject W is focused on a position sensor 505 by using LD lights of an LD 504. A signal of the position sensor 505 is processed in a focus detection circuit 506, and a focus control circuit 507 calculates a focus displacement amount to control the focus.
In addition, a method is known in which front focus and back focus states of an image to be focused are formed, and the focus position is calculated based on a difference of integration values (contrast) of light intensity of the front focus state and the back focus state. (Refer to, e.g., Japanese Published Unexamined Patent Application No. 11-271597)
However, according to the focus detection of the optical lever method mentioned above, when a critical dimension or a pitch of a semiconductor pattern becomes below a wavelength of the illumination light used in the focus detection, a diffracted light is generated at the pattern to be focused on the position sensor. Consequently, there is a problem that the correct focus position may not be detectable. Moreover, in the case of using an integration value of light intensity, when a difference of integration values is small, there is a problem that the focus position may not be detected accurately.